Autumnal Equinox -- In astronomy, is the equinox at the beginning of autumn in the northern hemisphere: the moment when the sun appears to cross the celestial equator, heading southward.
The equinox occurs around September 22-24, varying slightly each year according to the 400 year cycle of leap years in the Gregorian Calendar.
In the southern hemisphere, the equinox occurs at the same...

Orbit -- An orbit is the path that an object makes around another object under the influence of some force.
The classical example is that of the solar system, where the Earth, other planets, asteroids, comets, and smaller pieces of rubble are in orbit around the Sun; and moons are in orbit around planets. These days, many artificial satellites are in orbit around the Earth....

Escape Velocity -- An escape velocity is the minimum speed at which an object without propulsion can move away from a source of a gravitational field indefinitely if there is no friction.
This definition may need modification for the practical problem of two or more sources in some cases. In any case, the object is assumed to be a point with a mass that is negligible compared with that of...

Word of the Day

upstander

A person who stands up for something, as contrasted to a bystander who remains inactive.

One of the upright handlebars on a traditional Inuit sled.

The word 'upstander' in the first sense here is a play on the word 'bystander' and the idea of 'standing up' for something.